1. Field of the Invention
This invention relates to an ultrasonic motor in which progressive vibration waves are generated in a stator (fixed element) to drive a rotor (moving element) and which therefore has an improved in driving efficiency. That is, the present invention relates to an ultrasonic motor in which traveling vibration waves are generated in an elastic body by excitation of a piezoelectric element at a vibration frequency in an ultrasonic range to drive a rotor which is maintained in contact with the elastic body by being pressed against the same.
2. Related Background Art
Known ultrasonic motors have a type of structure such as the one disclosed in U.S. Pat. No. 4,743,788. FIG. 11 is a cross-sectional view of portions of a circular ring ultrasonic motor having such a structure.
A stator (fixed element) 1 has a ring-like elastic body la and a ring-like piezoelectric element 1b integrally bonded together. A rotor (moving element) 2 has a rotor base member 2a and a slider member 2b integrally bonded together. The rotor base member 2a has a flange 2c radially projecting from its portion in the vicinity of a neutral plane. A support (not shown) is formed integrally with the flange at an outer peripheral end of the same. A pressing force applied from a pressing member (not shown) is transmitted to the rotor 2 through the support to press the rotor 2 so that a lower surface (contact surface) of the slider member 2b contacts a drive surface 1c of the elastic body 1a. The slider member 2b, which is provided to improve the efficiency of driving of the rotor 2, is formed of a material having a large friction coefficient and high wear resistance.
In this construction, when an alternating current voltage is applied to the piezoelectric element 1b, the piezoelectric element 1b vibrates in a flexing manner to cause progressive vibration waves in the elastic body 1b in the circumferential direction, and the rotor 2 is driven by these vibration waves in a friction driving manner.
In this construction, however, the width b of the bonding surface 2d of the slider member 2b in the radial direction must be larger than a certain value because it is necessary to provide a bonding area between the slider member 2b and the rotor base member 2a large enough to prevent the slider member 2b and the rotor base member 2a from separating from each other during driving of the rotor 2. In the conventional ultrasonic motor, therefore, the width b of the bonding surface is selected to obtain a sufficient bonding strength. Accordingly, a contact surface 2e of the slider member 2b in contact with the elastic body 1a has a comparatively large width corresponding to the width b. The conventional ultrasonic motor therefore entails a drawback described below.
The amplitude, the wavelength and the circumferential speed of progressive vibration waves generated on the driving surface 1c of the elastic body 1a are dependent upon the distance from a center axis l of the rotor 2 corresponding to the center of rotation thereof, and the frictional driving force received by the slider member 2b from the elastic body 1a changes with respect to the radial position around the rotational center axis l. In a case where the radial width of the contact surface of the slider member in contact with the elastic member is large, as in the case of the conventional motor, the degree of non-uniformity of the frictional driving force between the inner and outer circumferential ends of the slider member 2b is substantially large, so that the slider member 2b and the elastic member 1a relatively slide on each other. The efficiency of driving of the ultrasonic motor is reduced by friction losses caused by this sliding.
A linear ultrasonic motor also entails a similar drawback. That is, if the width of a contact surface of a moving element in contact with a fixed element (size in the direction perpendicular to the direction in which progressive vibration waves travel) is large, frictional losses occur at different positions because of the non-uniformity of the frictional driving force distribution in the widthwise direction of the contact surface owing to the differences between contact states of the contact surface portions and non-uniformity of pressing force.